Packet-switching networks are commonly employed to transfer digital information over long distances. Packet-switching networks are also known as cell networks. A typical packet-switching network enables a variety of communication devices coupled to local communication links to share common carrier communication links. The packet-switching network enables the communication devices to transfer digital information on a demand driven basis. The demand driven sharing of the common carrier communication links reduces the cost of maintaining a long distance communication network.
For example, a packet-switching network can typically enable communication between devices coupled to local T1 communication links. A T1 communication link enables transmission of digital information according to the circuit-switching digital signal level one (DS1) communication protocol. Typically, a source communication controller coupled to the packet-switching network samples a local DS1 data stream, and assembles the local DS1 data stream into one or more communication packet streams. The source communication controller then transmits the communication packets over the packet-switching network.
Usually, a destination communication controller coupled to the packet-switching network receives the communication packets over the packet-switching network. The destination communication controller then disassembles the packets. The destination communication controller reconstructs the original local DS1 data stream or the channels of the original local DS1 data stream.
The throughput of a packet-switching network can be maximized if the error retry overhead of the network is minimized. A communication packet may occasionally be lost during transmission over the packet-switching network. Nevertheless, the reliability of modern packet-switching networks and common carrier facilities usually can hold such losses to an acceptable level.
However, circuit-switching communication formats that require bit alignment or byte alignment within a data stream are particularly vulnerable to the loss of a communication packet during transmission over the packet-switching network.
For example, DS0-A timeslots in a DS1 frame on a T1 communication link have a stuff bit, 6 data bits and a control bit. The control bit qualifies the data bits as either user data or control information. The source communication controller samples DS0-A timeslots, and assemble the DS0-A samples into a series of communication packets. The source communication controller can typically discard the stuff bit of each DS0-A sample in order to maximize bandwidth utilization of the packet-switching network if the line coding technique for the packet-switching network does not require a stuff bit to meet ones density requirements. However, the discarding of the stuff bit can result in a bit skewed arrangement of DS0-A samples within the communication packets. As a consequence, the destination communication controller cannot properly reconstruct the original DS0-A data stream if a communication packet is lost during transmission over the packet-switching network.
For another example, N.times.DS0 timeslots in a DS1 frame require that a particular sequence of DS0 timeslots be reconstructed by the destination controller. Moreover, DS0 timeslots supporting a 56 kbps data rate have 7 data bits and a stuff bit. The source communication controller can typically discard the stuff bit of each DS0 timeslot sample in order to maximize bandwidth utilization of the packet-switching network if the line coding technique for the packet-switching network does not require a stuff bit. However, the discarding of the stuff bit can result in a byte skewed arrangement of DS0 timeslot samples within the communication packets. If a communication packet is lost during transmission over the packet-switching network, the destination communication controller cannot reconstruct the proper sequence of DS0 timeslots. As a consequence, the destination communication controller cannot properly reconstruct the original N.times.DS0 data stream. A similar problem arises when transferring an N.times.64 kbps data stream in communication packets containing stuff bits for meeting a ones density requirement of the cell network.
One possible method for aligning the digital data stream across the packet-switching network requires transmission of sync packets over the packet-switching network. The sync packets signal the destination communication controller to align the reconstructed data stream. However, the sync packets must be periodically transmitted to ensure proper alignment. Unfortunately, the periodic transmission of the sync packets can reduce the throughput of the packet-switching network by absorbing available bandwidth. Moreover, the sync packets are also susceptible to transmission errors and losses over the packet-switching network.